The invention relates to a silicon-germanium bipolar transistor having a germanium concentration profile which is optimized for the purpose of reducing an operating-point-dependent variation of the current gain.
Silicon-germanium bipolar transistors, which are abbreviated as SiGeBTs, represent a further development of silicon bipolar transistors and are distinguished by special performance in the high-frequency range.
In the case of such SiGeBTs, a base region and an edge zone of an adjoining emitter region are alloyed with germanium (Ge) in a targeted manner. Through the use of a suitable Ge concentration profile, in that region of the transistor, the band gap between valence and conduction bands is smaller, with the effect that charge carriers traverse the base region more rapidly. Therefore, SiGeBTs have very high limiting frequencies and are particularly suitable for high-frequency applications. Such SiGeBTs are known, for example, from an article in IEEE Electron Device Letters, Vol. 4, 1993 by F. Crabbe et al..
In an SiGeBT, the base region is usually alloyed with from 10 to 25% by weight of Ge. The transition in the Ge concentration from the emitter to the base has a profile that rises either abruptly or approximately linearly. With regard to the high-frequency properties of SiGeBTs, a Ge concentration profile with an approximately linear rise has proved to be optimal.
That concentration profile has the disadvantage of causing the current gain to be greatly dependent on the collector current which is respectively flowing. That disadvantage can be attributed to the fact that the current gain of a transistor is related to the position of the space charge zone of the emitter/base pn junction, which is abbreviated as EB space charge zone. In that case, the following two factors are significant:
Firstly, the current gain of a transistor falls with a rising base charge, which is determined by the position of the space charge zone.
Secondly, the current gain is a negative exponential function of the band gap in the base, in particular at the base-side boundary of the EB space charge zone.
With a rising base-emitter voltage, the base-side boundary of the EB space charge zone migrates in the direction of the emitter so that, on one hand, the base charge rises and, on the other hand, at that boundary the band gap is enlarged due to the falling Ge concentration. Both effects lead to a major reduction in the current gain.
Since the collector current also rises with a rising base-emitter voltage, the current gain indirectly depends on the collector current or on the operating point of the transistor and falls sharply with a rising collector current. That dependence is undesirable since, as a result, the linearity of the transistor is impaired and the dynamic range of the transistor is limited.
It is accordingly an object of the invention to provide a silicon-germanium bipolar transistor with an optimized germanium profile, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and which reduces an operating-point-dependent variation of a current gain of an SiGeBT.
With the foregoing and other objects in view there is provided, in accordance with the invention, a silicon-germanium bipolar transistor, comprising a silicon substrate and, formed in the silicon substrate: a first n-doped emitter region having an edge zone, a second p-doped base region adjoining the emitter region and a third n-doped collector region adjoining the base region. A first space charge zone is formed between the emitter region and the base region and a second space charge zone is formed between the base region and the collector region. The base region and the edge zone of the adjoining emitter region are alloyed with germanium. The emitter region has a germanium concentration rising toward the base region. A junction region contains the first space charge zone and has a germanium concentration rising less sharply than in the emitter region or decreasing. The base region has a germanium concentration initially rising more sharply than in the junction region.
In the case of the transistor according to the invention, in a narrow region in the vicinity of the EB space charge zone, the Ge concentration profile deviates from the customary Ge profiles with an approximately linear rise in the Ge concentration. According to the invention, the Ge alloy is embodied in such a way that, in the event of a shift in the base-side boundary of the EB space charge zone, the factors which cause the change in the current gain largely compensate for one another.
The variation of the current gain becomes smaller, the smaller the rise in the Ge concentration in the direction of the collector and the further this rise is shifted in the direction of the collector. In the case of the Ge doping profiles according to the invention, the Ge concentration rises less sharply or falls toward the base in the vicinity of the EB space charge zone as compared with the adjoining regions.
In accordance with another feature of the invention, the Ge concentration is kept constant at a suitable level in the vicinity of the EB space charge zone.
In accordance with a further feature of the invention, the Ge profile is configured in such a way that the Ge concentration decreases in the vicinity of the EB space charge zone. In this case, the concentration gradient is chosen in such a way that the operating-point-dependent variation of the current gain is optimally compensated for.
In accordance with a concomitant feature of the invention, the emitter region and the collector region are p-doped and the base region is n-doped.
It is advantageous that, in the case of this invention, there is a reduction in the production-dictated manufacturing tolerance for SiGeBTs with regard to the nominal current gain.
This normally very large fluctuation in the nominal current gain of SiGeBTs is based on the fact that, due to unavoidable production tolerances, the exact position of the emitter-base pn junction relative to the Ge profile varies from transistor to transistor. In the case of the germanium profile configuration according to the invention, this production-dictated variation leads to smaller changes in the nominal current gain than in the case of SiGeBTs according to the prior art.
A further advantage of this invention is that the nominal current gain can be set very precisely due to the targeted alteration of the Ge concentration in the region of the emitter-base pn junction or the adaptation of the Ge concentration in this region to the maximum Ge content.
Since only a small region in the linear rise of the Ge profile is modified, the high-frequency properties of an SiGeBT are only marginally altered by the Ge profile configuration according to the invention.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a silicon-germanium bipolar transistor with an optimized germanium profile, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.